Pressure support devices are well known for supplying pressurized air to a patient by way of a nasal mask. So-called Continuous Positive Airway Pressure ("CPAP") devices have a variety of applications, including assisting respiration and as a treatment for sleep apnea. The positive pressure supplied by the ventilator assists the patient's inhalation, maintains the patency of the patient's airways, and inflates the patient's lungs to a resting volume higher than normal.
An improvement on the basic pressure support device is the bi-level pressure support ventilator, wherein periods of higher pressure are interspersed with periods of lower pressure. The lower pressure facilitates the patient's exhalation, as the patient need not overcome the higher pressure to expel a breath. There have been various approaches to providing bi-level support, including controlling the duration of the high and low pressure intervals by means of a timer, and detecting the patient's inhalation and exhalation, supplying higher pressure during the patient's inhalation and a lower pressure during exhalation.
A problem with some prior art pressure support devices has been that the devices are often perceived by the patient as objectionably noisy. Such ventilators provide positive pressure by means of a blower, with blower speed being rapidly varied to regulate the output pressure to the patient. Since the output pressure varies somewhat with flow, the blower speed was increased or decreased to maintain patient pressure against the disturbance of patient flow. The blower speed variations are exacerbated in those bi-level pressure systems, wherein the blower speed must be varied not only to maintain a constant pressure against the disturbance of patient flow but also to provide the intervals of higher and lower pressure. This rapid variation of the blower speed however, is readily detected by the patient as audible noise.
In addition, some prior art pressure support ventilators operate a blower at maximum speed and lower pressure by exhausting the excess air to atmosphere. This approach avoids the objectionable rapid variation of the blower speed. However, the constant operation of the blower at maximum speed and the venting of large volumes of air to the ambient serve to create an objectionably high noise level.
Thus there is a need for a pressure support device which can maintain a constant pressure against the disturbance of patient flow without the audible noise associated with varying the speed of the blower.
There is a further need for a pressure support device which can supply alternating periods of higher and lower pressures without the audible noise associated with varying the speed of a blower.
There is still another need for a pressure support device which can attain the desired patient pressures without operating its blower at maximum speed and without venting large volumes of air to the ambient.
Another problem associated with certain prior art bi-level pressure support ventilators concerns what is known as "rise time," that is, the time required for the patient pressure to rise from the lower pressure level to the higher pressure level. Rise time figures prominently into patient comfort, as a rise time which is too fast or too slow may not be well tolerated. To complicate matters, a rise time which may be comfortable for one patient may cause discomfort in another patient. For pressure support ventilators which modulate pressure by varying blower speed, it may be difficult to provide meaningful control over rise time.
Thus there is a need for a pressure support ventilator which permits adjustment of rise time to accommodate the comfort of the patient.
Certain prior art bi-level pressure support ventilators which coordinate pressure modulation with the patient's breathing detect inspiration and expiration by means of a flow sensor positioned between the pressure regulator valve and patient. However, locating the flow meter between the valve and patient subjects the flow meter to possible fluids in the patient hose. In addition, the characteristic resistance of the flow meter to a person trying to exhale might be objectionably high.
Thus there is a need for a pressure support ventilator which detects inspiration and expiration of the patient without subjcting the flow meter to possible fluids in the patient hose.
There is also a need for a pressure support ventilator which detects patient inspiration and expiration without a flow meter creating an objectionable resistance to patient exhalation.